GB2255372A

GB2255372A - I.c.engine spark ignition system

Info

Publication number: GB2255372A
Application number: GB9109400A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1991-05-01
Filing date: 1991-05-01
Publication date: 1992-11-04
Also published as: GB9109400D0; WO1992019862A1

Abstract

A distributorless ignition system is described for a spark ignited internal combustion engine which reduces the risk of spark plug fouling. In addition to the usual ignition circuit (10, 12, 14, 16) which generates sparks in synchronisation with the position of the crankshaft while the engine is running for initiating charge combustion, the system of the invention includes circuitry (18 to 26) for exciting the coil (16) to generate sparks before or after an engine run, for preventing the formation of a deposit on the spark plug or burning off any existing deposit.

Description

Title Ignition System Field of the invention The present invention is concerned an ignition system for a spark ignited internal combustion engine.

Background of the invention Spark plug fouling is a serious problem for the car manufacturer before the car is delivered to the customer.

This is because of the large number of cold starts and short runs in the course of assembly, testing, storage and transit of the vehicle. On average some fifty cold starts can occur for each car before it finally reaches the customer.

Because of this large number of cold starts and short runs without the opportunity for burning off any deposits on the spark plugs, the deposits tend to accumulate and cause the fouling referred to above, which prevents the spark plugs from firing correctly by short circuiting the electrode gap. This in turn causes misfiring which can have dire consequences in an engine fitted with a catalytic converter. Misfire in the engine cylinders results in fuel being passed unburned to the catalytic converted and the combustion of this unburned fuel in the catalytic converter may give off sufficient heat to cause melt down of the catalyst.

Object of the invention This invention seeks to minimise the risk of spark plug fouling when an engine is subjected to frequent cold starts and short runs.

Summary of the invention According to the present invention, there is provided a distributorless ignition system for a spark ignited internal combustion engine in which each spark plug is permanently connected to a respective ignition coil, first means for exciting the coil in synchronisation with the position of the crankshaft while the engine is running to generate sparks for initiating charge combustion, and second means for exciting the coil to generate sparks while the engine is not running, for preventing the formation of a deposit on the spark plug or burning off any existing deposit.

In a first embodiment of the invention, the second means is brought into operation for a predetermined period at the end of an engine run so as to maintain the spark plug hot and avoid fuel being deposited in the electrodes.

It is possible as an alternative, or in addition, to generate sparks for a predetermined period before the engine is cranked at start up, in order to reduce the risk of misfire by first cleaning and warming the electrodes.

The invention requires the ignition system to be of the distributorless type as otherwise only the spark plug connected to the ignition coil by the distributor at the time that the engine comes to a standstill could be cleaned or kept clean by sparking, as proposed in the present invention.

A simple oscillator, such as an astable multivibrator, suffices to excite the coils when the engine is not running. The oscillator may operate a switching circuit which cyclically connects the low tension winding of the coils to the vehicle battery then suddenly interrupts the connection. A monostable multivibrator may determine the length of time that the astable multivibrator oscillates.

Brief description of the drawing The invention will now be described further, by way of example, with reference to the accompanying drawing which is a block circuit diagram of an ignition system in accordance with the invention.

Detailed description of the preferred embodiment In the drawing, the blocks 10 to 16, inclusive, form a conventional ignition circuit and may be a mechanical, electro-optical or a fully electronic ignition system. In particular, the blocks 10 to 16 comprise a spark plug 16 connected to the high tension winding of a coil 14. The low tension winding of the coil 14 is connected to the vehicle battery (not shown) by a switching circuit 12 which comprises electronic switching elements (such as transistors or thyristors) controlled by the ignition timing circuit 10. The latter circuit receives a signal from the engine crankshaft from which it determines the crankshaft angle and sends switching pulses at appropriate crankshaft angles which are selected to take into account engine speed and load in a conventional manner.

The remaining blocks shown in the drawing are intended to create sparks at the spark plug 16 when the engine has been turned off after running. A retriggerable monostable multivibrator 18 has a duration longer than the maximum time between sparks when the engine is running. As a result, when the engine is running, the output of the circuit 18 is permanently high. This signal is fed to a relay 22 the change-over contact of which connects the ignition timing circuit 10 to the switching circuit 12 for as long as the engine is turning so that the circuitry provided for generating sparks after the engine has stopped does not interfere with the normal operation of the engine.

If the engine is stopped after having been cranked or after a run, the retriggerable monostable multivibrator 18 will eventually return to its stable state and its output will go low. This occur for example five seconds after the engine has come to a stop, this period being longer than the time taken for the engine to stop turning after being switched off.

The negative going transition when the output of the circuit 18 goes low is sensed by a trailing edge detector circuit 20, which may for example comprise a capacitor followed by an inverter, to produce a signal for triggering a timer 24, which may be a monostable multivibrator with a time of 10 sec. This is the time during which sparks will be produced at the spark plug 16.

The output of the timer 24 enables or gates an oscillator 26, which generates the individual sparks, the output of the oscillator being fed to the switching circuit 12 by way of the contact of the relay 22, which will have changed over when the retriggerable monostable multivibrator 18 reverted to its stable state.

It can be seen from the foregoing description that the invention can be implemented by placing a component containing the circuit blocks 18 to 26, between the blocks 10 and 12 of a conventional ignition circuit. Aside from enabling the invention to be realised by minor internal modification of existing ignition systems, it has the advantage that it can be manufactured as an accessory module which can be inserted and removed at will. The manufacturer may therefore choose to use the module only to avoid spark plug fouling prior to delivery, the module being removed on delivery to the customer. Thus, the manufacture can avoid a serious problem without adding to the cost of the equipment supplied to the customer.In normal use, the fouling problem is rare and the module is not needed, but it can of course be fitted as an accessory to vehicles which are used predominantly for short trips.

As so far described, the additional sparks while the engine is not turning have been generated after the engine has stopped but it is possible as an alternative, or in addition, to generate sparks before the engine is cranked.

This may either be done by incorporating a time delay in the starter motor relay to allow the sparks to be generated prior to the engine being cranked or, as in the case of the glow plugs of a diesel engine, instructing the driver to hold the ignition key for a few seconds before engaging the starter motor. It is a simple matter to detect that the ignition is on but that the engine is not turning in order to enable the oscillator 26 and changeover the contact of the relay 22.

For ease of understanding, the invention has been described by reference to a hardware implementation but it will also be clear to the person skilled in the art that the invention can be implemented in software where a vehicle is fitted with an ignition computer. In such computers, the computer receives signals from various sensors, computes the times at which sparking should occur and produces an output excitation pulse to trigger a spark at the desired moment. By suitable alteration of the program, it is possible to make the computer generate additional excitation pulses after the engine has been turned off or before it is cranked.

If sparks are generated before the engine is started, an additional advantage gained is that moisture or possibly even ice can be removed from the spark plug before fuel is introduced into the cylinder for combustion.

If additional sparks are generated after the engine has been switched off, one should ensure that the further sparking does not keep the engine running. One way of achieving this aim is to use the ignition key to control only the fuelling of the engine so that the injection system ceases to operate after the ignition key is turned off. In this way, fuel ceases to be supplied to the cylinders and the additional can have no effect other than the prevention of the build up of a deposit on the spark plug electrodes. Alternatively, one can ensure that the crankshaft has come to a complete halt before initiating the additional sparks for cleaning the spark plug gap.

Claims

1. A distributorless ignition system for a spark ignited internal combustion engine in which each spark plug is permanently connected to a respective ignition coil, first means for exciting the coil in synchronisation with the position of the crankshaft while the engine is running to generate sparks for initiating charge combustion, and second means for exciting the coil to generate sparks while the engine is not running, for preventing the formation of a deposit on the spark plug or burning off any existing deposit.

2. An ignition system as claimed in claim 1, wherein the second means is arranged to operate for a predetermined period at the end of an engine run so as to maintain the spark plug hot and avoid fuel being deposited on the electrodes.

3. An ignition system as claimed in claim 2, wherein the second means comprises means for detecting when the engine comes to a stop, and a timer set by the latter means for enabling an oscillator for a length of time determined by the timer, the output signal of the oscillator serving to cause excitation of the coil to generate sparks in the spark plug.

4. An ignition system as claimed in any preceding claim, wherein the second means is operative to generate sparks for a period before the engine is cranked at start up, in order to reduce the risk of misfire by first cleaning and warming the electrodes.

5. An ignition system constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.